1. Field of the Invention
The present invention relates to an apparatus for operating on picture element data representing a picture or a selected region of a picture as a plurality of picture elements, to obtain picture element data which represent that picture or selected region as a different number of picture elements.
More particularly the invention relates to a picture element data density conversion apparatus whereby such data can be converted to picture element data having a lower density, for thereby enabling the picture or selected region to be displayed or printed in compressed size with an arbitrarily selected degree of compression.
2. Description of the Prior Art
Various types of apparatus for effecting a reduction of picture element data density have been proposed in the prior art. Such an apparatus is applicable for example to a copier apparatus, to enable selective reduction in size of an image or selected region of an image that is copied.
In the prior art, one method of achieving such selective size variation of copies produced by a copier apparatus has been to use an optical lens having a zoom capability. However due to the requirement for a high degree of optical accuracy, use of such an optical lens in a copier apparatus will result in a substantial increase in the manufacturing cost. Furthermore it is inherently difficult to achieve a sufficiently high degree of performance from such a lens.
Other methods have therefore been proposed, which are based upon picture element data density variation, to achieve an arbitrary amount of change (in general, compression) of the size of an original picture or region of an original picture. It should be noted that the term "picture" is used herein in a very general sense, to include for example a picture that is conveyed by a field of a television signal, a printed page of text, etc. FIGS. 1A, 1B are simple diagrams to illustrate the basic concept of picture element data density conversion and picture size compression. In FIG. 1A, an array of picture elements of an original picture are represented by corresponding values D1, D2, . . . , D36, which have been obtained for example by optically scanning the picture. If the picture is to be printed out or displayed in compressed size, by comparison with the size which would be obtained by using the set of values D1 to D36, and assuming that the desired compression factor is 5/6 in the horizontal and vertical directions for example, then that is achieved by first converting the set of values D1 to D36 to a 2-dimensionally reduced-density set of values D1', D2', . . . , D25' as shown in FIG. 3. The derived reduced-density values may occur intermittently, i.e. may not be continuously sequential along the time axis, so that it is then necessary to execute time axis compression in order to convert the data to suitable form for obtaining a compressed-size picture. That is generally done by writing the density-reduced values into a buffer memory, then reading the values out in a continuous sequence.
One method which uses a hardware technique for reducing the data density of a digital picture element signal employs a line image sensor which produces the picture element data by scanning across an original picture, with the line image sensor being moved in a secondary (e.g. vertical) scanning direction while being scanned across the picture in the main (e.g. horizontal) scanning direction. A reduced picture element data density is thereby achieved, by comparison with operation in which the line image sensor is not moved in the secondary scanning direction while moving in the main scanning direction. However such a method presents basic problems in the case of a color scanning operation. In general, a color line image sensor is formed of respective sensors for the primary colors, which are arrayed in the secondary scanning direction, so that these normally move in parallel across the picture being scanned, in the main scanning direction. However if such a color line image sensor is moved in the secondary scanning direction while moving in the main scanning direction, it becomes extremely difficult to properly combine the respective color output signals produced from the color line image sensor, and it is found that spurious signal components arise.
Other methods have been proposed in the prior art whereby software operation, i.e. computer processing, is used to reduce the picture element data density of a signal representing a picture or a region of a picture. However with such prior art methods, it is found that a relatively large amount of processing time is required, so that it becomes impossible to achieve certain types of real-time operation, such as size compression of a selected region of a moving picture.